This invention relates generally to an electrically controlled shift actuator and an electrical control system therefor and more particularly to a shift actuator, preferably a gear shift actuator for an axle, of the type utilizing a rotatable member to position a shift member engagement means at one of two shift positions to effect the shift thereat or to preload the shift member engagement means for shifting thereof to a selective one of two selectable operating positions sufficiently to effect the shift on the occurrence of an event enabling the shift to occur.
Shift actuators, such as two-position shift actuators, for selectively shifting or positioning a shifting member, such as a shift fork or the like, are well known in the prior art. Shift actuators allowing a shift to each of at least two preselected positions by preloading an element, usually a resilient element such as spring, to bias the shift member, usually a shift fork or the like, towards each of the two preselected positions are also well known in the art. Examples of the latter devices are disclosed respectively in U.S. Pat. Nos. 2,754,692; 2,821,089 and 3,358,517, the disclosures of which are incorporated herein by reference.
The present invention relates to the control of shift actuators of the type utilizing a rotatable member for moving a shift member engagement means between at least two spaced-apart shift positions including the shift actuator disclosed in U.S. Pat. No. 4,428,248 issued Jan. 31, 1984, and assigned to the assignee of the present invention, the disclosure of which is included herein by reference. More particularly, the present invention relates to an electrical control of shift actuators of the aforementioned type which employ a selector switch for selectively connecting one of two position sensing switches to a power supply for connecting a drive motor for the rotatable member to the power supply through the respective position sensing switch, and means associated with said rotatable member for operating said position sensing switch in a prescribed sequence for disconnecting the motor from the power supply and connecting a dynamic braking energy dissipation means in circuit with the motor when the rotatable member reaches a predetermined rotational position such as is disclosed in U.S. Pat. No. 4,550,625 issued Nov. 5, 1985, and assigned to the assignee of the present invention, the disclosure of which is included herein by reference.
Although shifting actuators of the type utilizing an electrical control for connecting a dynamic braking energy dissipation circuit to the drive motor have been generally satisfactory, such control energizes the drive motor while the dynamic braking circuit is still temporarily connected to the motor, thereby drawing heavy start-up current through switch contacts. A large resistor may be included in the dynamic braking circuit to substantially match the motor armature resistance, thereby reducing current in the start-up condition, but this reduces the dynamic braking efficiency as well. The exposure of heavy start-up currents to contacts in switches of the type used for detecting or sensing the position of the shift mechanism accelerate erosion of the switch contacts and shorten switch life. Induction energy stored in the motor is manifested as arcing across the contacts of a position sensing switch when first opened to remove power from the motor, such arcing producing an undesirable EMI noise level.